Unsolved!: The History and Mystery of the World's Greatest Ciphers from Ancient Egypt to Online Secret Societies

From 1968 until the early 1970s, the San Francisco area was the scene of several murders committed by somebody who came to be known as the Zodiac Killer, a person who to this day has never been identified, much less captured. The Zodiac, as he referred to himself, taunted the public by sending messages to the media. Initially, three different newspapers received messages in the form of a cypher, each consisting of a rectangular array of symbols.

Although the CIA, FBI and NSA had been given these messages, it took a husband and wife team of amateurs, Donald and Bettye Harden, to crack them. Proceeding on the inspired assumption that an attention-seeker like The Zodiac would begin his message with the word “I”, and might even use the phrase “I LIKE KILLING”, the Hardens, assuming that the message involved substitution of symbols for letters (even though there were too many symbols used to be a simple substitution cypher), were able to decrypt the message. However, the Zodiac killer subsequently sent another cypher, and that one has not been cracked.

The book now under review, written by the author of the excellent Secret History: The Story of Cryptology, is filled with fascinating stories like this one, though not all as gruesome in their details (though several others also involve murders). The book discusses, in a story-telling, informative style, a variety of ciphers spanning several millennia. Most, but not all of them, remain unsolved: the title of this book notwithstanding, the author has included some solved ones (as in the example above) where doing so sheds light on the unsolved ones. In addition to the mechanics of the cipher itself, the author also provides a lot of background commentary, including many photographs and other illustrations.

Some of the ciphers discussed in the text may not in fact be ciphers at all. The author points out that some experts believe that the documents are just gibberish created by illiterates rather than a serious attempt to convey a message.

Space considerations make it impossible to thoroughly discuss all the various ciphers (or possible ciphers) that are discussed in this rather thick (more than 600 pages) book, so a representative sample will have to suffice. Unsolved begins with the famous book-length manuscript now called the Voynich manuscript (Voynich being the bookseller who rediscovered this in 1912) which dates back to the 15th century and consists of both print and pictures, the meaning of which is, to put it mildly, unclear.

In addition to this, the reader of this book will learn about messages (or potential messages) from space, a cipher found taped to the stomach of a person killed by cyanide poisoning (who also had on his person a fountain pen gun and a picture of Nazi aircraft), a cipher created by a likely serial killer who was hanged for the murder of his third wife (but may have also killed his first two), and ciphers involving possible buried treasure.

The author does not just summarize the history of these ciphers but also talks about possible solution methods and his own contributions to their investigation.

Some comparison with Bauer’s previous book, Secret History, seems appropriate. Many of the examples mentioned here are also discussed in Secret History, but in much less detail; the Zodiac ciphers occupy just a few pages in that book, for example. In addition, the mathematical level of this text is lower than that of Secret History; Unsolved has been deliberately written for a general audience, with (for the most part) little actual mathematics used. For some chapters, such as the one on RSA, a familiarity with some undergraduate-level mathematical knowledge would be helpful (although even here the author endeavors to develop everything from scratch, omitting proofs). By and large the exposition does not get even this mathematically detailed, and the reader is only expected to understand basic cryptological methods as they are explained and to follow specific examples.

Secret History, the author tells us, has been used as a text in two different kinds of cryptography courses (a freshman-level course for non-majors and a junior-level course for people with some prior exposure to mathematics), and although there are no exercises in that book, there are lots of them available online at the author’s website. There are no exercises in Unsolved (and no indication that there will be any posted online); for that reason, and because of its general structure, it seems not to be intended as a text for a traditional course in cryptography (though it might be fun to use it as a text for an honors seminar in unsolved ciphers).

It would also, I think, make excellent supplemental reading by students or faculty members in a standard cryptography course. It is clearly and invitingly written, told from a “story-teller” perspective, and contains a great deal of intrinsically interesting material that can be used to enhance lectures. And even a person who will never take, or teach, a course in which cryptography is discussed, should find that this book is just plain fun to read.

A few years back, I taught an undergraduate junior/level course in number theory. Naturally, I wanted to spend at least some time that semester discussing cryptography as one of the most significant and most interesting applications of number theory. Based on subsequent student comments, the cryptography component of the course was a big hit. I wish I had had this book on my desk when I taught that course; the students would, I think, have enjoyed the lectures even more.

Mark Hunacek (mhunacek@iastate.edu) teaches mathematics at Iowa State University.